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                    示例: 表达式求值
            
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                    并发的循环
            
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                    示例: 并发的Web爬虫
            
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                    示例: 并发的目录遍历
            
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                    并发的退出
            
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                    基于共享变量的并发
            
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                    竞争条件
            
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                    sync.Mutex互斥锁
            
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                    sync.Once惰性初始化
            
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                    竞争条件检测
            
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        <li class="chapter " data-level="1.10.7" data-path="../ch9/ch9-07.html">
            
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                    示例: 并发的非阻塞缓存
            
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                    Goroutines和线程
            
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                    包和工具
            
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                    包简介
            
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                    包声明
            
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                    导入声明
            
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                    包的匿名导入
            
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                    包和命名
            
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                    工具
            
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                    测试
            
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                <a href="../ch11/ch11-01.html">
            
                    
                    go test
            
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                    测试函数
            
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                    测试覆盖率
            
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                    基准测试
            
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                    剖析
            
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                    示例函数
            
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                    反射
            
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        <li class="chapter " data-level="1.13.2" data-path="../ch12/ch12-02.html">
            
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                    reflect.Type和reflect.Value
            
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        <li class="chapter " data-level="1.13.3" data-path="../ch12/ch12-03.html">
            
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                    Display递归打印
            
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        <li class="chapter " data-level="1.13.4" data-path="../ch12/ch12-04.html">
            
                <a href="../ch12/ch12-04.html">
            
                    
                    示例: 编码S表达式
            
                </a>
            

            
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        <li class="chapter " data-level="1.13.5" data-path="../ch12/ch12-05.html">
            
                <a href="../ch12/ch12-05.html">
            
                    
                    通过reflect.Value修改值
            
                </a>
            

            
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        <li class="chapter " data-level="1.13.6" data-path="../ch12/ch12-06.html">
            
                <a href="../ch12/ch12-06.html">
            
                    
                    示例: 解码S表达式
            
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        <li class="chapter " data-level="1.13.7" data-path="../ch12/ch12-07.html">
            
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                    获取结构体字段标签
            
                </a>
            

            
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                    显示一个类型的方法集
            
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                    几点忠告
            
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                    底层编程
            
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                    unsafe.Sizeof, Alignof 和 Offsetof
            
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        <li class="chapter " data-level="1.14.2" data-path="../ch13/ch13-02.html">
            
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                    unsafe.Pointer
            
                </a>
            

            
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                    示例: 深度相等判断
            
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        <a href=".." >flag.Value接口</a>
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                                <h2 id="74-flagvalue&#x63A5;&#x53E3;">7.4. flag.Value&#x63A5;&#x53E3;</h2>
<p>&#x5728;&#x672C;&#x7AE0;&#xFF0C;&#x6211;&#x4EEC;&#x4F1A;&#x5B66;&#x5230;&#x53E6;&#x4E00;&#x4E2A;&#x6807;&#x51C6;&#x7684;&#x63A5;&#x53E3;&#x7C7B;&#x578B;flag.Value&#x662F;&#x600E;&#x4E48;&#x5E2E;&#x52A9;&#x547D;&#x4EE4;&#x884C;&#x6807;&#x8BB0;&#x5B9A;&#x4E49;&#x65B0;&#x7684;&#x7B26;&#x53F7;&#x7684;&#x3002;&#x601D;&#x8003;&#x4E0B;&#x9762;&#x8FD9;&#x4E2A;&#x4F1A;&#x4F11;&#x7720;&#x7279;&#x5B9A;&#x65F6;&#x95F4;&#x7684;&#x7A0B;&#x5E8F;&#xFF1A;</p>
<p><u><i>gopl.io/ch7/sleep</i></u></p>
<pre><code class="lang-go"><span class="hljs-keyword">var</span> period = flag.Duration(<span class="hljs-string">&quot;period&quot;</span>, <span class="hljs-number">1</span>*time.Second, <span class="hljs-string">&quot;sleep period&quot;</span>)

<span class="hljs-keyword">func</span> main() {
    flag.Parse()
    fmt.Printf(<span class="hljs-string">&quot;Sleeping for %v...&quot;</span>, *period)
    time.Sleep(*period)
    fmt.Println()
}
</code></pre>
<p>&#x5728;&#x5B83;&#x4F11;&#x7720;&#x524D;&#x5B83;&#x4F1A;&#x6253;&#x5370;&#x51FA;&#x4F11;&#x7720;&#x7684;&#x65F6;&#x95F4;&#x5468;&#x671F;&#x3002;fmt&#x5305;&#x8C03;&#x7528;time.Duration&#x7684;String&#x65B9;&#x6CD5;&#x6253;&#x5370;&#x8FD9;&#x4E2A;&#x65F6;&#x95F4;&#x5468;&#x671F;&#x662F;&#x4EE5;&#x7528;&#x6237;&#x53CB;&#x597D;&#x7684;&#x6CE8;&#x89E3;&#x65B9;&#x5F0F;&#xFF0C;&#x800C;&#x4E0D;&#x662F;&#x4E00;&#x4E2A;&#x7EB3;&#x79D2;&#x6570;&#x5B57;&#xFF1A;</p>
<pre><code>$ go build gopl.io/ch7/sleep
$ ./sleep
Sleeping for 1s...
</code></pre><p>&#x9ED8;&#x8BA4;&#x60C5;&#x51B5;&#x4E0B;&#xFF0C;&#x4F11;&#x7720;&#x5468;&#x671F;&#x662F;&#x4E00;&#x79D2;&#xFF0C;&#x4F46;&#x662F;&#x53EF;&#x4EE5;&#x901A;&#x8FC7; -period &#x8FD9;&#x4E2A;&#x547D;&#x4EE4;&#x884C;&#x6807;&#x8BB0;&#x6765;&#x63A7;&#x5236;&#x3002;flag.Duration&#x51FD;&#x6570;&#x521B;&#x5EFA;&#x4E00;&#x4E2A;time.Duration&#x7C7B;&#x578B;&#x7684;&#x6807;&#x8BB0;&#x53D8;&#x91CF;&#x5E76;&#x4E14;&#x5141;&#x8BB8;&#x7528;&#x6237;&#x901A;&#x8FC7;&#x591A;&#x79CD;&#x7528;&#x6237;&#x53CB;&#x597D;&#x7684;&#x65B9;&#x5F0F;&#x6765;&#x8BBE;&#x7F6E;&#x8FD9;&#x4E2A;&#x53D8;&#x91CF;&#x7684;&#x5927;&#x5C0F;&#xFF0C;&#x8FD9;&#x79CD;&#x65B9;&#x5F0F;&#x8FD8;&#x5305;&#x62EC;&#x548C;String&#x65B9;&#x6CD5;&#x76F8;&#x540C;&#x7684;&#x7B26;&#x53F7;&#x6392;&#x7248;&#x5F62;&#x5F0F;&#x3002;&#x8FD9;&#x79CD;&#x5BF9;&#x79F0;&#x8BBE;&#x8BA1;&#x4F7F;&#x5F97;&#x7528;&#x6237;&#x4EA4;&#x4E92;&#x826F;&#x597D;&#x3002;</p>
<pre><code>$ ./sleep -period 50ms
Sleeping for 50ms...
$ ./sleep -period 2m30s
Sleeping for 2m30s...
$ ./sleep -period 1.5h
Sleeping for 1h30m0s...
$ ./sleep -period &quot;1 day&quot;
invalid value &quot;1 day&quot; for flag -period: time: invalid duration 1 day
</code></pre><p>&#x56E0;&#x4E3A;&#x65F6;&#x95F4;&#x5468;&#x671F;&#x6807;&#x8BB0;&#x503C;&#x975E;&#x5E38;&#x7684;&#x6709;&#x7528;&#xFF0C;&#x6240;&#x4EE5;&#x8FD9;&#x4E2A;&#x7279;&#x6027;&#x88AB;&#x6784;&#x5EFA;&#x5230;&#x4E86;flag&#x5305;&#x4E2D;&#xFF1B;&#x4F46;&#x662F;&#x6211;&#x4EEC;&#x4E3A;&#x6211;&#x4EEC;&#x81EA;&#x5DF1;&#x7684;&#x6570;&#x636E;&#x7C7B;&#x578B;&#x5B9A;&#x4E49;&#x65B0;&#x7684;&#x6807;&#x8BB0;&#x7B26;&#x53F7;&#x662F;&#x7B80;&#x5355;&#x5BB9;&#x6613;&#x7684;&#x3002;&#x6211;&#x4EEC;&#x53EA;&#x9700;&#x8981;&#x5B9A;&#x4E49;&#x4E00;&#x4E2A;&#x5B9E;&#x73B0;flag.Value&#x63A5;&#x53E3;&#x7684;&#x7C7B;&#x578B;&#xFF0C;&#x5982;&#x4E0B;&#xFF1A;</p>
<pre><code class="lang-go"><span class="hljs-keyword">package</span> flag

<span class="hljs-comment">// Value is the interface to the value stored in a flag.</span>
<span class="hljs-keyword">type</span> Value <span class="hljs-keyword">interface</span> {
    String() <span class="hljs-keyword">string</span>
    Set(<span class="hljs-keyword">string</span>) error
}
</code></pre>
<p>String&#x65B9;&#x6CD5;&#x683C;&#x5F0F;&#x5316;&#x6807;&#x8BB0;&#x7684;&#x503C;&#x7528;&#x5728;&#x547D;&#x4EE4;&#x884C;&#x5E2E;&#x52A9;&#x6D88;&#x606F;&#x4E2D;&#xFF1B;&#x8FD9;&#x6837;&#x6BCF;&#x4E00;&#x4E2A;flag.Value&#x4E5F;&#x662F;&#x4E00;&#x4E2A;fmt.Stringer&#x3002;Set&#x65B9;&#x6CD5;&#x89E3;&#x6790;&#x5B83;&#x7684;&#x5B57;&#x7B26;&#x4E32;&#x53C2;&#x6570;&#x5E76;&#x4E14;&#x66F4;&#x65B0;&#x6807;&#x8BB0;&#x53D8;&#x91CF;&#x7684;&#x503C;&#x3002;&#x5B9E;&#x9645;&#x4E0A;&#xFF0C;Set&#x65B9;&#x6CD5;&#x548C;String&#x662F;&#x4E24;&#x4E2A;&#x76F8;&#x53CD;&#x7684;&#x64CD;&#x4F5C;&#xFF0C;&#x6240;&#x4EE5;&#x6700;&#x597D;&#x7684;&#x529E;&#x6CD5;&#x5C31;&#x662F;&#x5BF9;&#x4ED6;&#x4EEC;&#x4F7F;&#x7528;&#x76F8;&#x540C;&#x7684;&#x6CE8;&#x89E3;&#x65B9;&#x5F0F;&#x3002;</p>
<p>&#x8BA9;&#x6211;&#x4EEC;&#x5B9A;&#x4E49;&#x4E00;&#x4E2A;&#x5141;&#x8BB8;&#x901A;&#x8FC7;&#x6444;&#x6C0F;&#x5EA6;&#x6216;&#x8005;&#x534E;&#x6C0F;&#x6E29;&#x5EA6;&#x53D8;&#x6362;&#x7684;&#x5F62;&#x5F0F;&#x6307;&#x5B9A;&#x6E29;&#x5EA6;&#x7684;celsiusFlag&#x7C7B;&#x578B;&#x3002;&#x6CE8;&#x610F;celsiusFlag&#x5185;&#x5D4C;&#x4E86;&#x4E00;&#x4E2A;Celsius&#x7C7B;&#x578B;&#xFF08;&#xA7;2.5&#xFF09;&#xFF0C;&#x56E0;&#x6B64;&#x4E0D;&#x7528;&#x5B9E;&#x73B0;&#x672C;&#x8EAB;&#x5C31;&#x5DF2;&#x7ECF;&#x6709;String&#x65B9;&#x6CD5;&#x4E86;&#x3002;&#x4E3A;&#x4E86;&#x5B9E;&#x73B0;flag.Value&#xFF0C;&#x6211;&#x4EEC;&#x53EA;&#x9700;&#x8981;&#x5B9A;&#x4E49;Set&#x65B9;&#x6CD5;&#xFF1A;</p>
<p><u><i>gopl.io/ch7/tempconv</i></u></p>
<pre><code class="lang-go"><span class="hljs-comment">// *celsiusFlag satisfies the flag.Value interface.</span>
<span class="hljs-keyword">type</span> celsiusFlag <span class="hljs-keyword">struct</span>{ Celsius }

<span class="hljs-keyword">func</span> (f *celsiusFlag) Set(s <span class="hljs-keyword">string</span>) error {
    <span class="hljs-keyword">var</span> unit <span class="hljs-keyword">string</span>
    <span class="hljs-keyword">var</span> value <span class="hljs-keyword">float64</span>
    fmt.Sscanf(s, <span class="hljs-string">&quot;%f%s&quot;</span>, &amp;value, &amp;unit) <span class="hljs-comment">// no error check needed</span>
    <span class="hljs-keyword">switch</span> unit {
    <span class="hljs-keyword">case</span> <span class="hljs-string">&quot;C&quot;</span>, <span class="hljs-string">&quot;&#xB0;C&quot;</span>:
        f.Celsius = Celsius(value)
        <span class="hljs-keyword">return</span> <span class="hljs-literal">nil</span>
    <span class="hljs-keyword">case</span> <span class="hljs-string">&quot;F&quot;</span>, <span class="hljs-string">&quot;&#xB0;F&quot;</span>:
        f.Celsius = FToC(Fahrenheit(value))
        <span class="hljs-keyword">return</span> <span class="hljs-literal">nil</span>
    }
    <span class="hljs-keyword">return</span> fmt.Errorf(<span class="hljs-string">&quot;invalid temperature %q&quot;</span>, s)
}
</code></pre>
<p>&#x8C03;&#x7528;fmt.Sscanf&#x51FD;&#x6570;&#x4ECE;&#x8F93;&#x5165;s&#x4E2D;&#x89E3;&#x6790;&#x4E00;&#x4E2A;&#x6D6E;&#x70B9;&#x6570;&#xFF08;value&#xFF09;&#x548C;&#x4E00;&#x4E2A;&#x5B57;&#x7B26;&#x4E32;&#xFF08;unit&#xFF09;&#x3002;&#x867D;&#x7136;&#x901A;&#x5E38;&#x5FC5;&#x987B;&#x68C0;&#x67E5;Sscanf&#x7684;&#x9519;&#x8BEF;&#x8FD4;&#x56DE;&#xFF0C;&#x4F46;&#x662F;&#x5728;&#x8FD9;&#x4E2A;&#x4F8B;&#x5B50;&#x4E2D;&#x6211;&#x4EEC;&#x4E0D;&#x9700;&#x8981;&#x56E0;&#x4E3A;&#x5982;&#x679C;&#x6709;&#x9519;&#x8BEF;&#x53D1;&#x751F;&#xFF0C;&#x5C31;&#x6CA1;&#x6709;switch case&#x4F1A;&#x5339;&#x914D;&#x5230;&#x3002;</p>
<p>&#x4E0B;&#x9762;&#x7684;CelsiusFlag&#x51FD;&#x6570;&#x5C06;&#x6240;&#x6709;&#x903B;&#x8F91;&#x90FD;&#x5C01;&#x88C5;&#x5728;&#x4E00;&#x8D77;&#x3002;&#x5B83;&#x8FD4;&#x56DE;&#x4E00;&#x4E2A;&#x5185;&#x5D4C;&#x5728;celsiusFlag&#x53D8;&#x91CF;f&#x4E2D;&#x7684;Celsius&#x6307;&#x9488;&#x7ED9;&#x8C03;&#x7528;&#x8005;&#x3002;Celsius&#x5B57;&#x6BB5;&#x662F;&#x4E00;&#x4E2A;&#x4F1A;&#x901A;&#x8FC7;Set&#x65B9;&#x6CD5;&#x5728;&#x6807;&#x8BB0;&#x5904;&#x7406;&#x7684;&#x8FC7;&#x7A0B;&#x4E2D;&#x66F4;&#x65B0;&#x7684;&#x53D8;&#x91CF;&#x3002;&#x8C03;&#x7528;Var&#x65B9;&#x6CD5;&#x5C06;&#x6807;&#x8BB0;&#x52A0;&#x5165;&#x5E94;&#x7528;&#x7684;&#x547D;&#x4EE4;&#x884C;&#x6807;&#x8BB0;&#x96C6;&#x5408;&#x4E2D;&#xFF0C;&#x6709;&#x5F02;&#x5E38;&#x590D;&#x6742;&#x547D;&#x4EE4;&#x884C;&#x63A5;&#x53E3;&#x7684;&#x5168;&#x5C40;&#x53D8;&#x91CF;flag.CommandLine.Programs&#x53EF;&#x80FD;&#x6709;&#x51E0;&#x4E2A;&#x8FD9;&#x4E2A;&#x7C7B;&#x578B;&#x7684;&#x53D8;&#x91CF;&#x3002;&#x8C03;&#x7528;Var&#x65B9;&#x6CD5;&#x5C06;&#x4E00;&#x4E2A;<code>*celsiusFlag</code>&#x53C2;&#x6570;&#x8D4B;&#x503C;&#x7ED9;&#x4E00;&#x4E2A;flag.Value&#x53C2;&#x6570;&#xFF0C;&#x5BFC;&#x81F4;&#x7F16;&#x8BD1;&#x5668;&#x53BB;&#x68C0;&#x67E5;<code>*celsiusFlag</code>&#x662F;&#x5426;&#x6709;&#x5FC5;&#x987B;&#x7684;&#x65B9;&#x6CD5;&#x3002;</p>
<pre><code class="lang-go"><span class="hljs-comment">// CelsiusFlag defines a Celsius flag with the specified name,</span>
<span class="hljs-comment">// default value, and usage, and returns the address of the flag variable.</span>
<span class="hljs-comment">// The flag argument must have a quantity and a unit, e.g., &quot;100C&quot;.</span>
<span class="hljs-keyword">func</span> CelsiusFlag(name <span class="hljs-keyword">string</span>, value Celsius, usage <span class="hljs-keyword">string</span>) *Celsius {
    f := celsiusFlag{value}
    flag.CommandLine.Var(&amp;f, name, usage)
    <span class="hljs-keyword">return</span> &amp;f.Celsius
}
</code></pre>
<p>&#x73B0;&#x5728;&#x6211;&#x4EEC;&#x53EF;&#x4EE5;&#x5F00;&#x59CB;&#x5728;&#x6211;&#x4EEC;&#x7684;&#x7A0B;&#x5E8F;&#x4E2D;&#x4F7F;&#x7528;&#x65B0;&#x7684;&#x6807;&#x8BB0;&#xFF1A;</p>
<p><u><i>gopl.io/ch7/tempflag</i></u></p>
<pre><code class="lang-go"><span class="hljs-keyword">var</span> temp = tempconv.CelsiusFlag(<span class="hljs-string">&quot;temp&quot;</span>, <span class="hljs-number">20.0</span>, <span class="hljs-string">&quot;the temperature&quot;</span>)

<span class="hljs-keyword">func</span> main() {
    flag.Parse()
    fmt.Println(*temp)
}
</code></pre>
<p>&#x4E0B;&#x9762;&#x662F;&#x5178;&#x578B;&#x7684;&#x573A;&#x666F;&#xFF1A;</p>
<pre><code>$ go build gopl.io/ch7/tempflag
$ ./tempflag
20&#xB0;C
$ ./tempflag -temp -18C
-18&#xB0;C
$ ./tempflag -temp 212&#xB0;F
100&#xB0;C
$ ./tempflag -temp 273.15K
invalid value &quot;273.15K&quot; for flag -temp: invalid temperature &quot;273.15K&quot;
Usage of ./tempflag:
  -temp value
        the temperature (default 20&#xB0;C)
$ ./tempflag -help
Usage of ./tempflag:
  -temp value
        the temperature (default 20&#xB0;C)
</code></pre><p><strong>&#x7EC3;&#x4E60; 7.6&#xFF1A;</strong> &#x5BF9;tempFlag&#x52A0;&#x5165;&#x652F;&#x6301;&#x5F00;&#x5C14;&#x6587;&#x6E29;&#x5EA6;&#x3002;</p>
<p><strong>&#x7EC3;&#x4E60; 7.7&#xFF1A;</strong> &#x89E3;&#x91CA;&#x4E3A;&#x4EC0;&#x4E48;&#x5E2E;&#x52A9;&#x4FE1;&#x606F;&#x5728;&#x5B83;&#x7684;&#x9ED8;&#x8BA4;&#x503C;&#x662F;20.0&#x6CA1;&#x6709;&#x5305;&#x542B;&#xB0;C&#x7684;&#x60C5;&#x51B5;&#x4E0B;&#x8F93;&#x51FA;&#x4E86;&#xB0;C&#x3002;</p>

                                
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